An R-T-B-based sintered magnet including an R2T14B type compound as a main phase (R is composed of a light rare-earth element(s) RL and heavy rare-earth element(s) RH, RL is Nd and/or Pr, RH is at least one of Dy, Tb, Gd and Ho, and T is a transition metal element and inevitably includes Fe) has been known as a permanent magnet with the highest performance among permanent magnets, and has been used in various motors for hybrid cars, electric cars and home appliances.
However, in the R-T-B-based sintered magnet, coercive force HcJ (hereinafter sometimes simply referred to as “HcJ”) decreases at a high temperature to cause irreversible thermal demagnetization. Therefore, when used particularly in motors for hybrid cars and electric cars, there is a need to maintain high HcJ even at a high temperature. In addition, there is a need to obtain higher HcJ at room temperature so as not to cause irreversible thermal demagnetization at a high temperature.
To increase HcJ, a large amount of heavy rare-earth elements (mainly, Dy) have hitherto been added to the R-T-B-based sintered magnet. However, there arose a problem that a residual magnetic flux density Br (hereinafter sometimes simply referred to as “Br”) decreases. Therefore, there has recently been employed a method in which heavy rare-earth elements are diffused from the surface into the inside of the R-T-B-based sintered magnet to thereby increase the concentration of the heavy rare-earth elements at the outer shell part of main phase crystal grains, thus obtaining high HcJ while suppressing a decrease in Br.
Dy has problems such as unstable supply or price fluctuations because of restriction of the producing district. Therefore, there is a need to develop technology for improving HcJ of the R-T-B-based sintered magnet without using heavy rare-earth elements such as Dy as much as possible (by reducing the amount as much as possible).
Patent Document 1 discloses that the amount of B is decreased as compared with a conventional R-T-B-based alloy and one or more metal elements M selected from among Al, Ga, and Cu are included to form a R2T17 phase, and a volume fraction of a transition metal-rich phase (R6T13M) formed from the R2T17 phase as a raw material is sufficiently secured to obtain an R-T-B-based rare-earth sintered magnet having high coercive force while suppressing the content of Dy. Patent Document 1 also discloses a method for producing an R-T-B-based rare-earth sintered magnet in which a sintered body after sintering is subjected to a heat treatment at two temperatures of 800° C. and 500° C. and cooling.
Patent Document 2 specifies the effective amount of rare-earth elements and the effective amount of boron, and discloses an alloy containing Co, Cu, and Ga has higher coercive force HcJ at the same residual magnetization Br than a conventional alloy. Patent Document 2 also discloses a method for producing an Nd—Fe—B permanent magnet in which a sintered body after sintering is subjected to a heat treatment at 400° C. to 550° C.